In conventional rotary drilling systems such as are used in the oil and gas industry, a drill string is formed by connecting drill pipes together and mounting a drill bit at the lower end of the drill string. The bit is rotated by rotating the drill string and, by applying weight to the bit, is caused to drill ahead through the underground formations. Drilling fluid (‘mud’) is pumped down the inside of the drill string, out through holes in the bit then back up the annulus transporting drilled cuttings back to the surface. The mud also serves to lubricate the bit and balance the pressure of pore fluids in the formation drilled until it can be lined or cased.
In reverse circulation, mud is pumped down the annulus and into the inside of the drill string through the drill bit to return to the surface. In certain cases, crossover subs have been proposed to divert flow from the drill string into the annulus near the bit and/or return flow from the annulus into the inside of the drill string.
In cases where there is limited annular space (as may be the case in coiled tubing drilling, for example), reverse circulation may be preferred since the inside of the drill string may provide a better path for mud and cuttings than the narrow annulus that might become easily blocked by the cuttings.
It has recently been proposed to drill using a bottom hole assembly (‘BHA’) with an electrically powered drilling tool for driving the bit powered via a wireline cable rather than a conventional drill string. In this case, the preferred circulation is down the annulus across the toolface and radially into the center of the bit then back up through the inside of the BHA in view of the limited annulus in the region of the bit.
However, in this scenario the centripetal forces on the fluid caused by the rotation of the bit are acting against the direction of flow of the fluid, increasing the bit pressure drop as the rotary speed increases. In drilling tests at rotary speeds above 200 rpm the cuttings were forced out to the outer edge of the bit, by the centripetal forces, and were not convected out from the bit with the drilling fluid. In one set of tests, the pressure drop increased from about 1.7 psi to about 3.2 psi as the rotary speed was increased from 50 to 750 rpm.
It is an object of the invention to provide a system that avoids both the problem of a limited annulus and of the rotating bit when transporting drilled cuttings.